1. Field of the Invention
The invention relates to a jitter measuring method and device, and more particularly to a jitter measuring method and device for precisely calculating the length of a serial digital signal according to a multi-phase clock to measure the jitter correctly.
2. Description of the Related Art
In an optical storage apparatus, a jitter measuring device (jitter meter) is an important device. The measurement result of the jitter measuring device is an indicator for the signal quality. If a jitter measuring device with precise result is employed in the optical storage apparatus, the optical storage apparatus can acquire correct signal information and adjust itself to an optimum state. The jitter measuring method is typically divided into a digital method and an analog method.
The digital method includes the steps of shaping the serial digital signal, calculating the number of pulses of a reference clock for each pulse of the shaped signal, and then finding out a difference value between the calculated numbers as the jitter value. The drawbacks of this method are that it is necessary to provide the reference clock with higher frequency if the frequency of the serial digital signal is high.
The analog method includes the steps of converting each pulse width of the serial digital signal into an analog signal, and then filtering the voltage of the analog signal by a filter. The filtered voltage variation represents the jitter value. The drawbacks of this method are that the switching speed of the used switches may greatly influence the measurement result, and the switches with a high switching speed cannot be easily implemented.
In view of the above-mentioned problems, an object of the invention is to provide a jitter measuring method and device capable of precisely measuring jitters in a serial digital signal without a high-frequency reference clock.
To achieve the above-mentioned object, the jitter measuring device of the invention includes a rough length measuring unit, a multi-phase signal generator, a phase error measuring unit, a length integrating unit, a pulse selecting unit, an average length calculating unit, a length difference calculating unit, and a jitter calculating unit. The rough length measuring unit receives a serial digital signal and a reference clock and generates a rough pulse length for each pulse of the serial digital signal. The multi-phase signal generator generates a plurality of multi-phase clocks according to the reference clock. The phase error measuring unit receives the serial digital signal, the reference clock and the multi-phase clocks, and generates a positive edge phase error and a negative edge phase error. The length integrating unit receives the rough pulse length, the positive edge phase error and the negative edge phase error, and computes a pulse length for each pulse of the serial digital signal. The pulse selecting unit selects the pulse lengths as selected pulse lengths according to a length selection signal. The average length calculating unit receives the selected pulse lengths and computes an average pulse length of the selected pulse lengths. The length difference calculating unit receives the selected pulse lengths and the average pulse length and computes length differences between the selected pulse lengths and the average pulse length. The jitter calculating unit receives the length differences and computes an average of the length differences as a jitter.
Since the jitter measuring device of the invention measures the jitter in the serial digital signal according to the reference clock having lower frequency, the need for providing a reference clock having high frequency can be avoided.